There are now many empty filter rings available, so that users who bring their own filters do not have to remove the standard filters from their rings. The standard filters have their name (BVRgri) engraved on the side of the filter ring.
The filter rings are 4 inches in diameter. There are also a set of five adapters to mount 2-inch square filters into the filter rings. Larger filter adapters need to be custom made.
Inserting and removing the filters from the instrument is a difficult process, so it is required that you be instructed by a member of the day crew or an experienced observer. It is necessary to use a clear plastic device to insert/remove the filters; this tool is kept in the black carrying cases. In the prime focus cage, you should move one-quarter way around the cage in the clockwise direction. At waist level is the entrance to the ``pizza box'', which has a clasp to open/close it. It is usually easiest to squat or sit on the floor of the cage in order to insert/remove filters, but note that there is a thin metal lever arm that holds each filter in place that is at eye level when you sit on the floor of the cage. For this reason, you should always wear protective glasses when inserting/removing filters.
In order to move the filter wheel, you need to press the appropriate buttons on the blue motor controller box in the prime focus cage. The filter wheel can be incremented by pressing the ``FILTER'' button followed by the ``INC'' button. There is an alternative ``DEC'' button to decrement the filter wheel. In order to install a filter, you need to press ``FILTER'' followed by ``INSTALL''; this moves the filter wheel by a half-position, allowing access to a given filter. Since the installation position is on the opposite side of the filter wheel from the focal plane, the filter being installed is ``filter position plus four modulo seven.''
Each filter position MUST have a filter ring in place in order to turn the filter wheel. There are many spare rings, so if you want a clear aperture, just insert a spare ring.
You should attempt to have the filter wheel roughly balanced - in other words, do not put filters in slots 1-4 and empty rings in slots 5-7, but alternate filters and empty rings.
Always press ``FILTER'' on the blue motor controller box when finished installing or removing a filter, in order to move the filter wheel from a half-position (installation) to an integer position (observing). Not doing this will prevent the COSMIC program on the Sparc computer from successfully moving the filter wheel. NEVER move the filter wheel by hand!